1. Field of the Invention
This invention relates to engine control systems in general and more particularly to a closed loop spark control system and algorithm operating around borderline knock spark timing and using the characteristics of a sensor related to combustion quality to determine the borderline knock spark timing.
2. Description of the Related Art
Spark timing in current production engines may be under open loop or closed loop control. If open loop, the spark timing is determined by calibrated look-up tables and calculated algorithms based on overall engine operating parameters and control inputs. If this method is used, the spark timing must be conservative (retarded) enough in order that the engine will not knock under worst-case conditions of compression ratio, fuel octane, combustion chamber deposits, and humidity. Since worst-case conditions are rare, retarded spark timing most or possibly all of the time will artificially reduce engine torque/efficiency.
To regain this torque/efficiency and maintain protection under worst-case conditions, many programs put the spark timing under closed loop control with a knock detection system. In some cases the same open loop spark tables are used to determine spark advance, and if knock is detected spark is retarded from that spark advance. Spark is then advanced if no further knocking events are detected until the original reference is reached. If more knocking events are detected, then spark is retarded further.
In other cases, the spark timing look-up tables contain upper and lower limits for different overall engine operating parameters, and feedback from the knock detector is used to find the optimal spark advance within those limits. The desired setpoint is the limit of knock audibility or the threshold of knock-induced piston damage or maximum engine torque, whichever comes first.
In both closed-loop cases the control strategy is arranged so that the knock detection system classifies each combustion event as knocking or non-knocking. If a combustion event is classified as non-knocking, the spark timing is advanced or maintained for the next engine cycle. However, if a combustion event is classified as knocking, the spark timing is retarded for the next engine cycle. There are many implementations of such strategies which differ in the details of how far and how fast the spark is advanced or retarded as well as how the knocking/non-knocking classification is made. Some systems maintain running averages and statistical measures of the individual cycle results for self-calibration purposes, and it is common to treat the knocking and non-knocking cycles differently in these statistical calculations.
U.S. Pat. No. 4,527,524 issued to Guipaud on Jul. 9, 1985, teaches the amount of spark retard used is based on the number of knock events detected and the rate used to advance spark after a knock event. Less spark retard is used with each successive knock event. Spark is advanced at an increasing rate if knock is not detected as the original spark advance is reached. This system relates strictly to spark advance/retard strategy with no mention of how an event is declared knocking or non-knocking. There is no attempt to correlate spark advance to actual borderline knock spark timing value.
U.S. Pat. No. 4,971,007 issued on Nov. 20, 1990 to Gopp et al. and assigned to a common assignee, is concerned both with finding the knock threshold and finding "Minimum spark for Best Torque", MBT. The knock threshold is detected by counting the number of engine cycles between successive knocking events. The system is not designed to optimize knock threshold accuracy.
These and other prior art systems do not attempt to correlate knock to actual borderline spark advance values. In addition the prior art systems are not designed to optimize knock threshold accuracy.